The present invention relates to a transmission system for multicasting data from a primary station via a network to a plurality of secondary stations, the primary station being arranged for collecting feedback information from the secondary stations, the feedback information being related to a state of reception of the data by the secondary stations.
The present invention also relates to a primary station for multicasting data via a network to a plurality of secondary stations, to a secondary station for receiving data via a network from a primary station and to a method of multicasting data from a primary station via a network to a plurality of secondary stations.
A transmission system according to the preamble is known from the paper entitled “Scalable Feedback Control for Multicast Video Distribution in the Internet” as published in the Proceedings of the ACM SIGCOMM '94, p. 58-67. Multicasting offers the possibility to transmit identical data to a large number of receivers in a more efficient way than via unicast connections. In the known transmission system a primary station multicasts video data packets via the Internet network to a plurality of secondary stations. After a number of data packets have been transmitted feedback information about the state of the network is solicited from the secondary stations. With this feedback information the rate at which the primary station transmits the data packets into the network can be adjusted so that congestion of the network can be prevented.
However, by soliciting information from a large number of secondary stations the network may become congested as many of the secondary stations simultaneously transmit feedback information to the primary station. In the known transmission system this so-called feedback implosion problem is tackled by eliciting the feedback information from the secondary stations in a series of rounds of probing. At the start of the series of rounds of probing both the primary station and the secondary stations generate random 16-bit keys. The primary station transmits its key in a feedback request to the secondary stations. The feedback request also comprises a number indicating the how many of the bits of the key are significant. Initially, all digits are significant. If the key generated by the primary station equals the key generated by a secondary station, using the declared number of significant bits, this secondary station transmits its local perception of the state of the network to the primary station. If there is no response within a timeout period set at twice the largest round trip time in the group of receivers, the number of significant bits is reduced and the primary station transmits a new feedback request comprising the same key and the reduced number of significant bits. This procedure is repeated until a state of the network is known by the primary station or until the feedback request was transmitted to the secondary stations with the number of significant bits being equal to zero.
In the known transmission system correction information is transmitted by the primary station to the secondary stations so that the secondary stations can correct any erroneously received data packets. However, this is only done when the number of secondary stations is relatively low so that there is little danger of feedback implosion. Furthermore, the feedback information in this case only consists of a negative acknowledgement by a secondary station that a data packet has been lost. After receipt of that negative acknowledgement the primary station encodes the blocks that were in that data packet using intramode coding and transmits the resulting correction information to the secondary station involved.
Despite these measures is the probability that an implosion of the feedback information occurs in the known transmission system still relatively high.